Discharge light source with electron beam excitation

ABSTRACT

A light source has a discharge vessel which is filled with a filling gas, and an electron beam source which is arranged in vacuum or in a region of low pressure. The electron beam source generates electrons which are propelled through an entry foil into the discharge vessel. An electric field may be generated inside the discharge vessel.

The invention relates to a light source with a discharge vessel which isfilled with a filling gas, and with an electron beam source arranged invacuum or in a region of low pressure, which source generates electronsand propels them through an inlet foil into the discharge vessel.

Such a light source is known from U.S. Pat. No. 6,052,401. A rare gas ispresent in the discharge vessel. The electrons propelled into thevessel, also denoted primary electrons hereinafter, have a high kineticenergy and knock second electrons, also denoted secondary electronshereinafter, present in outer shells of atoms away from the atoms. Aprimary electron introduced through the foil is capable of knockingsecondary electrons from several atoms in succession in cascade fashionbefore losing its kinetic energy. The rare gas ions change into excitedrare gas molecules, denoted excited state dimers or excimers for short,after several reaction steps. Such an excimer decomposes spontaneouslyand emits ultraviolet or UV radiation during this. Atoms of the gas arethus ionized by the introduced electrons, the ionization energy beingfinally converted into a UV photon. The efficacy of the electron beamsource and of the light source is low.

The invention accordingly has for its object to provide an improvedlight source. In particular, the efficacy, i.e. the ratio of generatedlight to the power consumed, is to be improved.

According to the invention, an electric field can be generated insidethe discharge vessel. Further processes develop inside the dischargevessel in addition to the process described above.

Secondary electrons are free electrons which have elastic collisionswith one another and with the atoms of the filling gas. An energybalance of the electrons adjusts itself automatically in a very shorttime, which balance can be described by approximation by means of aMaxwell velocity distribution. The electron temperature T_(e) heredenotes the average kinetic energy of the electrons. A proportion ofthese free electrons has a kinetic energy which is sufficient forexciting atoms. Free electrons collide with electrons present in atomshells and transfer their kinetic energy, which they build up againsubsequently through acceleration in the electric field. The electronsof the atoms absorb the energy and jump to an outer shell with a higherenergy level. The shells are numbered consecutively starting in thecenter towards the outside. When an electron drops back to a lowerenergy level, energy is released in the form of radiation.

The injected electrons thus substantially initiate two physicallydifferent processes. On the one hand atoms are ionized, while on theother hand atoms are excited. The two processes require differentquantities of energy. The electron temperature T_(e) adjusts itselfindependently by means of the ionization. This temperature, however, isnot an optimum for an efficient excitation of the atoms. Since theionization requires a substantially higher energy than the excitation,the electron temperature is too high for an efficient excitation, andthus for an efficient generation of UV radiation. When the gas insidethe discharge vessel, also denoted gas volume hereinafter, is exposed tothe electric field, the electron temperature T_(e) required for anefficient excitation may be freely chosen across the electric field. Theelectron beam serves essentially for generating the charge carriers inthe gas volume and the preliminary configuration of an ionized gas, alsodenoted plasma hereinafter. The application of an electric field acrossthe gas volume additionally induces a glow discharge. Light is emittedin principle because of the electric field. The power for generating theelectron beam is reduced.

In a simple embodiment, the discharge vessel comprises electricallyconductive electrodes. The electrodes generate the electric field in acapacitive manner. Electrodes arranged inside the discharge vessel maybe operated with an AC or a DC voltage, those arranged outside thedischarge vessel are operated with an AC voltage.

Advantageously, the electrodes comprise a dielectric. A frequency of theAC voltage may be reduced because of the dielectric.

In a simple embodiment, the discharge vessel comprises a coil. The coilgenerates an inductive AC field.

Advantageously, the discharge vessel comprises a microwave resonator.The microwave resonator generates a rotational field which causeselectrons to rotate along circular paths.

Advantageously, the electron beam source comprises a field emitter. Afield emitter array, a surface emitter array, or an array of nanotubesmay be used for this. Very small constructional units can be achievedthereby. The arrays have a grid-type structure or a surface comprisingpyramids or tentacles, from whose tips electrons are freed.

A conventional electron gun as used in TV tubes may also be used forgenerating the electron beam. The electron gun must be operated in highvacuum so as to avoid a destruction of the cathode by ionized residualgases.

Advantageously, the filling gas comprises at least one of the rare gasesHe, Ne, Ar, Kr, Xe. Being ionizable, rare gases serve to generate lightin the UV range and serve as a buffer gas for generating chargecarriers.

Advantageously, the filling gas comprises at least one of the gases H₂,N₂, O₂, F₂, Cl₂.

Advantageously, the filling gas comprises at least one of the followingelements which are wholly or partly evaporated under operationalconditions: Br, I, S, Se, Te, Po, P, As, Sb, Zn, Cd, Hg, In, Tl, Li, Na,K, Rb, Cs, Sr, and Ba in atomic or molecular form. Suitable fillinggases are in particular pure rare gases or mixtures of a rare gas and alight-emitting gas. If pure rare gases are used, a very efficient way ofgenerating, for example, UV radiation is by means of excimer radiation.If a mixture of light-emitting gas and rare gas is used, for exampleargon/mercury, a lamp with a high brightness is obtained. Alternativelight-emitting gases, however, are molecular radiators which may bechemically highly aggressive because of the absence of inner electrodes.The gases emit visible light, UV, or infrared radiation. A hugeadvantage arises from the possibility that the states excited by theelectron beam can be utilized for further excitation in the electricfield. For example, the ions generated in the electron beam may beutilized for further excitation in the electric field. There is aplurality of ions, for example Ba⁺, Rb⁺, or Cs⁺, which have strongtransitions in the visible wavelength range. The same principle isoperative in the excitation from long-life excited states which aregenerated in large numbers by the electron beam. A simple example is theneon gas mentioned above, whose first excited level in the third shell,also denoted 3s level hereinafter, is caused to be occupied by means ofthe electron beam. Starting from this level, which has a very longeffective life because the decay into the base state is stronglyhampered by reabsorption in the dense neon gas, a plurality of higherlevels may be excited by the electric field, which levels willsubsequently emit radiation in the visible wavelength range. Apossibility of having the 3s level of neon occupied to an even greaterextent is offered in a mixture of helium and neon. Starting from thehelium ion generated in large numbers by the electron beam, it ispossible here to occupy finally the 3s state of the neon via a series ofprocesses. A system based on the electron beam and the applied field maybe readily arranged such that only a small portion of the electricalenergy supplied on the outer side is used for the electron beam, forexample 10%, whereas the major portion of the energy, 90% in this case,is utilized for the efficient radiation generation in the electricfield.

Advantageously, the discharge vessel comprises a phosphor which convertssaid UV radiation into visible light.

Advantageously, the discharge vessel comprises two diametrically opposedmirrors. The mirrors form optical resonance bodies with parallel orslightly concave surfaces and serve to generate coherent light for alaser.

Advantageously, the electron beam source is operated in a pulsatorymanner. The pulsatory operation serves to generate coherent light for alaser.

An embodiment will be explained in more detail below for betterunderstanding with reference to the drawing, in which:

FIG. 1 shows a light source with external electrodes in cross-sectionalview,

FIG. 2 shows a second light source with a microwave resonator incross-sectional view, and

FIG. 3 shows a third light source with internal electrodes incross-sectional view.

FIG. 1 shows a light source 1, also denoted gas discharge lamphereinafter, with an electron beam source 2 and an electrode arrangement3 for generating a glow discharge. Electrons 4 are emitted from a heatedcathode 5 and pass through a hole 6 of a Wehnelt cylinder 7 into theacceleration range 8. Here the electrons 4 are accelerated towards aring anode 9 which they pass with an energy of 20 keV. Subsequently,they pass through a 300 nm thin entry window 10 of SiN into a gas space11 of the discharge vessel 12, also denoted gas container hereinafter.The electrons 4 lose no more than 10% of their energy when passingthrough the SiN window 10, the remainder is deposited, bounded stronglylocally, in the gas space 11 which is filled with 200 mbar pure neon. Abeam current amounts to approximately 0.1 mA. Each beam electrongenerates a plurality of secondary electrons and ions, i.e.approximately 500, in the gas space 11, and in addition a large numberof highly excited states. Two planar electrodes 13 and 14 are providedoutside the gas container 12, between which electrodes a radio-frequencyAC field with a frequency of 13.6 MHz and an average voltage of 500 V isapplied. The secondary electrons oscillate substantially in theradio-frequency AC field and support a discharge current, which isaccordingly approximately 500 times higher than the beam current of anelectron beam 15, i.e. approximately 50 mA. Approximately 25 W isaccordingly capacitively coupled into the plasma, while the electronbeam 15 introduces 2 W. The oscillating electrons adjust a uniformelectron temperature by means of elastic collisions, which temperaturehardly changes over a cycle because of the high frequency. The electrontemperature of the secondary electrons is so low here, owing to the lowratio between electric AC field strength and neon pressure, that saidsecondary electrons do not contribute to the ionization, but only to anefficient excitation starting from the long-life excited neon states,and thus to the efficient light generation. Since the electron beam 15introduces negative charge into the discharge vessel 12, this charge isto be drained off through a grounded wire 16 which is fused into thevessel 12.

A further embodiment of this system could be as follows: a cubicdischarge vessel 12 with an edge length of 5 cm is filled with 500 mbarhelium and 50 mbar neon. An electron beam 15 operates with 0.1 mA and 20kV, which corresponds to a power of 2 W. Each primary electron 4generates approximately 500 secondary electrons and secondary ions, i.e.the discharge current in the glow discharge is approximately 500 timesthe beam current, i.e. approximately 50 mA. The glow discharge has anaverage current of 50 mA and an average voltage of 500 V, whichcorresponds to a power of 25 W. At this very low ratio between electricfield and gas pressure of 0.25 V/(cm Torr), there is hardly anyionization caused by the glow discharge; the glow discharge has a stablepositive characteristic.

A third embodiment operates with 100 mbar argon and 5 mg mercury in thegas space 11, which has a volume of 3×3×3 cm³. The discharge vessel 12is heated so strongly in the stationary state that the mercury vaporpressure is approximately 1 mbar. Each beam electron generates mainly aplurality of more than 500 argon ions and secondary electrons in the gasspace 11. Outside the vessel 12, two planar, transparent electrodes 13and 14 of indium-tin oxide are provided, between which a radio-frequencyAC field with a frequency of 27 MHz is applied. The secondary electronsoscillate substantially in the radio-frequency AC field and carry thedischarge current, which is approximately 400 mA in this case. Theaverage voltage across the discharge is approximately 50 V. Accordingly,approximately 20 W is coupled capacitively into the plasma, while theelectron beam 15 contributes 2 W. The oscillating electrons adjust ahomogeneous electron temperature by means of elastic collisions, whichtemperature hardly changes over a cycle because of the high frequency.The electron temperature of the secondary electrons is so low, becauseof the low ratio between electric field strength and argon pressure,that said electrons contribute not to the ionization, but to theexcitation of the mercury and thus to the efficient generation of UVradiation at 254 nm. The conversion efficacy of the glow discharge powerinto UV radiation is 70%. A high brightness can be achieved because ofsmall constructional dimensions. If visible light is desired, a phosphormay be provided on the inside of the discharge vessel for converting theUV radiation. The electron beam generates the charge carriers in the gasvolume, keeps the glow discharge even, and leads to an immediateignition of the discharge. The mercury may be replaced by an alternativelight-generating gas whose vapor pressure is at least a few mTorr in thestationary state. Particularly interesting in this respect are, forexample, sodium, strontium, and barium, because these atoms have stronglines in the visible wavelength range, and in addition especiallymolecular radiators such as indium bromide, whose resonant radiationlies in or close to the visible wavelength range.

FIG. 2 shows a second light source 20 with the electron beam source 2,the gas space 11, and a microwave resonator 21 which induces a glowdischarge from the outside at a frequency of 2.45 GHz. The inducedelectric field is a rotational field; the electrons oscillate alongcircular path segments.

FIG. 3 shows a third light source 30 with the electron beam source 2, anelectrode arrangement 31, and a discharge vessel 32. Electrodes 33 and34 of the electrode arrangement 31 are formed as the cathode 33 andanode 34, respectively, which project into the discharge vessel 32. Thecathode 33 comprises a tungsten coil 35, the anode 34 comprises a planarmetal plate 36. Supply wires 37, 38, and 39 to the electrodes 33 and 34are fused into the discharge vessel 32. A DC voltage of 500 V is appliedbetween the cathode 33 and the anode 34. The cathode 33 is made to glowby an auxiliary heating current. The secondary electrons are the maincarriers of the glow discharge current, which is accordinglyapproximately 500 times stronger than the beam current, i.e.approximately 50 mA. The secondary electrons drift towards the anode 34in the electric field and thus adjust a very low electron temperature.Since the electron density should be approximately equal to the iondensity in the discharge volume, while the electron current is muchstronger than the ion current owing to the higher mobility of theelectrons in the electric field, electrons must be additionally suppliedfrom the cathode 33. This is achieved by the auxiliary heating of thecathode 33. An additional grounded wire is not necessary, because thefunction thereof is already performed by the anode 34.

LIST OF REFERENCE NUMERALS

-   1 light source-   2 electron beam source-   3 electron arrangement-   4 electrons-   5 cathode-   6 hole-   7 Wehnelt cylinder-   8 acceleration range-   9 ring anode-   10 entry window-   11 gas space-   12 discharge vessel-   13 electrode-   14 electrode-   15 electron beam-   16 wire-   20 light source-   21 microwave resonator-   30 light source-   31 electrode arrangement-   32 discharge vessel-   33 cathode-   34 anode-   35 tungsten coil-   36 planar metal plate-   37 supply wire-   38 supply wire-   39 supply wire

1. A light source comprising: a discharge vessel which is filled with a filling gas, an electron beam source arranged in vacuum or in a region of low pressure, wherein said electron beam source is configured to generate and propel electrons through an inlet foil into the discharge vessel, a pair of electrodes located at sides of the discharge vessel adjacent to the inlet foil, wherein at least one of the electrode is a coil configured to generate an inductive AC field in the discharge vessel, and an electric field generator configured to generate an electric field inside the discharge vessel between the pair of electrodes.
 2. The light source as claimed in claim 1, wherein the electrodes comprise a dielectric.
 3. The light source as claimed in claim 1, wherein the discharge vessel comprises a microwave resonator configured to generate a rotational field to cause the electrons to rotate along circular paths.
 4. The light source as claimed in claim 1, wherein the electron beam source comprises a field emitter.
 5. The light source as claimed in claim 1, wherein the filling gas comprises at least one of the rare gases He, Ne, Ar, Kr, Xe.
 6. The light source as claimed in claim 1, wherein the filling gas comprises at least one of the gases H₂, N₂, O₂, F₂, Cl₂.
 7. The light source as claimed in claim 1, wherein the filling gas comprises at least one of the following elements which are wholly or partly evaporated under operational conditions: Br, I, S, Se, Te, Po, P, As, Sb, Zn, Cd, Hg, In, Tl, Li, Na, K, Rb, Cs, Sr, and Ba in atomic or molecular form.
 8. The light source as claimed in claim 1, wherein the discharge vessel comprises a phosphor.
 9. The light source as claimed in claim 1, wherein the discharge vessel comprises two diametrically opposed mirrors.
 10. The light source of claim 1, wherein the electron beam source comprises an array having tips to facilitate freeing of the electrons.
 11. The light source of claim 10, wherein the array includes at least one of nanotubes, pyramids and tentacles.
 12. A light source comprising: a discharge vessel filled with gas; an electron beam source configured to generate and propel electrons through an inlet foil into the discharge vessel; a pair of electrodes located at sides of the discharge vessel adjacent to the inlet foil, wherein at least one of the electrode is a coil configured to generate an inductive AC field in the discharge vessel; and an electric field generator configured to generate an electric field inside the discharge vessel between the pair of electrodes.
 13. The light source of claim 12, wherein the electric field generator operates with an AC or a DC voltage.
 14. The light source of claim 12, wherein the electric field includes a rotational field.
 15. The light source of claim 12, wherein the electrons are configured to excite the gas into an excited state, and wherein the electric field is configured to further excite the excited state.
 16. The light source of claim 12, wherein a field current supplied to the electric field generator is approximately five hundred times higher than a beam current supplied to the electron beam source.
 17. The light source of claim 12, wherein a first energy supplied to the electron beam source is less than a second energy supplied to the electric field generator.
 18. The light source of claim 12, wherein a ratio of the electric field and pressure of the gas is reduced so that secondary electrons generated in the discharge vessel contribute more to excitation that to ionization of the gas.
 19. The light source of claim 12, wherein an electron current in the discharge vessel is higher than an ion current in the discharge vessel.
 20. The light source of claim 12, wherein an electron density in the discharge vessel is approximately equal to an ion density in the discharge vessel.
 21. The light source of claim 12, wherein the electron beam source comprises an array having tips to facilitate freeing of the electrons.
 22. The light source of claim 21, wherein the array includes at least one of nanotubes, pyramids and tentacles. 